I want to never hear about 3596 warnings. Can I tell it to
never warn me about those? I know I can filter the warnings list in
the problem panel, but I still get the little triangles on the
gutter of my editor..

In the project properties dialog (under Flex Compiler)
there's a checkbox labelled "Enable warnings" which should let you
kill the whole mess of them at once.

For a more targeted approach, you can add things to the
"Additional compiler arguments" that restrict which types of
warnings you see. Running "mxmlc -help list" on the command line
will list all of your options. You may need to restart FlexBuilder
after setting these for the first time.